1. Field of the Invention
This invention relates to processes for obtaining valuable fuel products from coal, and more particularly, relates to the integration of coal liquefaction and coal gasification so as to obtain a wide range of selected fuel products. Specifically, the invention relates to the processing of coal or other solid fuel products by solvent extraction of coal under supercritical conditions of temperature and pressure and coal gasification which are combined with a methanol-to-gasoline conversion process to produce a wide product slate of fuels.
Coal is becoming an increasingly attractive source for gaseous and liquid fuel inasmuch as coal is available in abundant supply and can be liquefied by a variety of techniques to produce a range of gaseous, distillate and nondistillate liquid coal products. It is recognized that the coal products may furnish a substitute for petroleum-based fuels and for petroleum-based feedstocks for the chemical industry.
It has also been well established that coal can be converted to gasoline by gasification of the coal and the subsequent production of methanol from the synthesis gas which is produced and the catalytic conversion of the methanol to gasoline. A ZSM-5 type zeolite catalyst has been found to be very effective in the conversion of methanol to gasoline. However, if a wide range of distillate products is desired from coal, the methanol-to-gasoline conversion process alone is not sufficient. Accordingly, a need exists to provide a wider product slate from coal than is ordinarily obtained from the methanol to gasoline conversion process. In accordance with the present invention, a wider product slate is obtained from coal by integrating the methanol-to-gasoline conversion process with coal liquefaction and coal gasification. As well, a very flexible, material and energy efficient coal conversion process is provided which allows greater selectivity as to the fuel products derived.
2. Description of the Prior Art
Deriving a gaseous and liquid fuel from coal utilizing integrated coal liquefaction and coal gasification is known to the art. For example, the selective extraction of hydrogen-rich constituents from coal using an organic solvent under supercritical conditions and gasification of the remaining charge is described by R. R. Maddocks et al "Coal Processing Technology: Supercritical Extraction of Coal", CEP, June, 1979. As described in the article, the chemical structure of the coal extract is such that it may be hydrocracked to distillable oils. The char or unextracted solid coal residue is described as an attractive feedstock for gasification and combustion and can provide the hydrogen required for extract hydrocracking, fuel gas for process heat and power, and surplus gas for electricity for sale.
A similar integrated process for deriving fuels from coal is disclosed in an article entitled "Development of a Process for the Supercritical Gas Extraction of Coal" by J. C. Whitehead, National Coal Board, Coal Research Establishment, Stoke Orchard, Cheltenham, England, 1979. In the article is described a process for deriving fuels from coal which includes the supercritical gas extraction of coal in which the extraction process is based on the ability of compressed gas to dissolve significant quantities of a high molecular weight substrate. The coal extract can be further hydrogenated. The article maintains that a variety of process options, in terms of processing routes and product slates, have been evaluated and that the majority of these options are based on the principal of generating power, process heat, and hydrogen from the residual solid char which remains after coal extraction. Any char excess to requirements in the schemes is converted to synthesis gas. Solvent make up for the supercritical gas extraction can be obtained from the products of extract hydrogenation.
U.S. Pat. No. 4,191,700, issued Mar. 4, 1980, to Lebowitz et al discloses a process for upgrading fuels, particularly coal, by means of integrating coal liquefaction and gasification and methanol synthesis. In this patent, coal is solvent refined with a conventional hydrogen donor solvent under severe conditions, preferably in a hydrogen environment, to convert substantially all the coal to a liquid product, which is divided in a vacuum still separation zone into a light distillate product, a recycle solvent, a heavy distillate, and a vacuum residue slurry. The vacuum residue slurry provides an efficient feed for a partial oxidation gasifier which produces synthetic gas as a feed for methanol and/or methane production and to supply hydrogen, as required, to the liquefier.
Although integrated coal liquefaction and coal gasification, as described above, is known and is used to derive a wide slate of fuel products from coal efficiently and with increased product selection, the integrated coal liquefaction and gasification processes, up to the present, have not fully utilized the char which remains after solvent extraction of the coal as described in the Maddocks et al and Whitehead articles or the heavy distillates and vacuum slurry separated from the coal conversion products as in the patent to Lebowitz et al to further increase the production of high value fuel products and optimize the ability to select which products are to be obtained from the coal. As set forth in the respective integrated processes discussed above, either the char which remains may be gasified to produce a synthesis gas or the vacuum residue slurry is converted to methanol or methane. The synthesis gas and methanol products are apparently used to provide heat for the respetive processes in which the excess products may be sold for heating value.